Electrical and optical cables, and especially continuously formed electrical and optical cables, are often encased in a seamless polymeric ("plastic") sheath or conduit of indeterminate length which provides a durable enclosure especially useful in subterranean applications. The resulting assembly is fabricated by pulling the cables through an extruder apparatus in which a plastic conduit is extruded about the longitudinal extent of the cables, the outer diameter of the plastic conduit then being sized by calibrator sizing rings positioned immediately downstream of the extruder head, followed by cooling and solidification of the conduit by passage of the assembly through serially-connected water cooling tanks.
Depending on the rate of extrusion and the rate of cooling of the conduit, the cables can contact the still-tacky inner surface of the plastic conduit resulting in the cables sticking to the inner surface as they travel with the extruded conduit. It is sometimes desirable to remove and replace the cables inside the conduit without replacing the conduit, especially in the case of underground installations. Accordingly, removal of the cables by pulling them from the plastic conduit would be prevented if the cables were adhered to the inner surface of the conduit.
Various approaches have been taken to overcome this problem. U.S. Pat. No. 3,211,818 discloses an extruder apparatus for making a conduit and cable assembly. The apparatus has a rigid tubular mandrel cantilevered from the extruder in the downstream direction of the extruded plastic conduit. The cantilevered tube supports the cables to be encased by the conduit and extends into a coolant sprayhead area a distance which is sufficient to allow for substantial solidification of the plastic conduit before the inner surface thereof is contacted by the cables. However, when the extrusion rate is increased, the cantilevered tube must be lengthened commensurately since solidification of the conduit and elimination of the tackiness of the inner surface of the conduit takes place farther downstream. When the cantilevered tube is lengthened, it begins to sag under its own weight causing the free end thereof to bear against the inner of the unsolidified conduit and resulting in gouging of the conduit wall of the conduit.
Another approach is disclosed in U.S. Pat. No. 4,508,500 in which a cable conduit extrusion apparatus includes a flexible corrugated metal tube extending from the extruder outlet into the extruded conduit as it travels through a differential pressure calibrating and cooling tank. The flexible tube has an outside diameter substantially the same as the inside diameter of the extruded conduit and is thereby supported along a substantial portion of the conduit on the peaks of the corrugations of the flexible tube. The flexible tube is primarily intended to accommodate up and down movement of the conduit resulting from buoyant forces acting on the conduit during its travel through the cooling tank. This apparatus still results in undesirable contact between the corrugated tube and unsolidified portions of the extruded conduit.
It would be desirable therefore to provide an apparatus for extruding a conduit containing one or more cables in which the cables and all other parts of the apparatus are kept separated from the inner wall of the conduit until the conduit is completely solidified regardless of the extrusion speed of the apparatus.